Xinzhe Li

MOF derived Co3O4 nanoparticles embedded in N-doped mesoporous carbon layer/MWCNT hybrids: extraordinary bi-functional electrocatalysts for OER and ORR

Highly efficient and non-precious metal electrocatalysts for oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) are at the heart of key renewable-energy technologies. Nevertheless, developing highly active bi-functional catalysts at low cost for both OER and ORR still remains a huge challenge. In this paper, Co3O4 nanocrystals embedded in N-doped mesoporous graphitic carbon layer/multiwalled carbon nanotube (MWCNT) hybrids are prepared by a facile carbonization and subsequent oxidation process of MWCNT-based metal–organic frameworks (MOFs). As a result, in alkaline media, the hybrid material catalyzes OER with an onset potential of 1.50 V (vs. reversible hydrogen electrode) and an over-potential only of 320 mV to achieve a stable current density of 10 mA cm−2 for at least 25 h. The same hybrids also exhibit similar catalytic activity but superior stability to the commercial 20 wt% Pt/C catalyst for ORR, making it a high-performance cheap bi-catalyst for both OER and ORR. The design concept of nonmetal-doped and precious-metal-free electrocatalysts from MOFs can be extended to fabricate other novel, stable and easy to use catalyst systems for advanced applications.

Nitrogen-doped mesoporous carbon nanosheet/carbon nanotube hybrids as metal-free bi-functional electrocatalysts for water oxidation and oxygen reduction

The development of metal-free catalysts for efficient catalysis of both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is extremely desirable in energy technologies. Herein, nitrogen-doped mesoporous carbon nanosheet/carbon nanotube (CNT) hybrids have been synthesized by the pyrolysis of glucose, urea and CNTs. Impressively, in 0.1 M KOH, the resulting hybrids afford remarkable OER activities with a low onset potential (1.50 V vs. RHE) and an exceptional over-potential (only 320 mV at 10 mA cm−2). Moreover, the same hybrids show comparable catalytic performance but better durability compared to the benchmark Pt/C (20 wt%) catalyst for ORR. The achieved ultrahigh catalytic performance of the hybrids originates from their large specific surface area (594.1 m2 g−1), high content percentage of N doping (8.5 wt%), and mesoporous structure, which leads to fully exposed active sites, improved mass/electron transport capability, easy adsorption/release of oxygen gas bubbles, and high structural stability. This work also provides a novel concept for fabricating heteroatom doped porous carbonaceous materials with integrated and improved catalytic performance for advanced applications.

MoS2 quantum dot decorated RGO: a designed electrocatalyst with high active site density for the hydrogen evolution reaction

Active, stable and cost-effective electrocatalysts are the key to water splitting for hydrogen production through electrolysis. In this work, we report MoS2 quantum dots (MoS2 QDs) decorated on reduced graphene oxide (RGO) synthesized by a facile sonication method as highly effective electrocatalysts for the hydrogen evolution reaction (HER). Compared with MoS2 sheets, the zero-dimensional MoS2 QDs have a defect-rich structure rendering these quantum dots with plentiful active sites, which can further enhance the catalytic activity by a synergistic effect with RGO. Electrochemical experiments demonstrated that the catalyst exhibited large cathode currents (a small overpotential of 64 mV for 10 mA cm−2 current density) and a Tafel slope as small as 63 mV per decade, achieving high stability simultaneously. This work opens up possibilities for preparing non-noble metal electrocatalysts while achieving high HER performance similar to commercial Pt catalysts (Pt/C).

Precious-metal-free Co–Fe–Ox coupled nitrogen-enriched porous carbon nanosheets derived from Schiff-base porous polymers as superior electrocatalysts for the oxygen evolution reaction

Water splitting provides a potential path for producing clean, renewable H2 and O2. However, improving the overall efficiency of water splitting is a challenging issue. Here, we designed Co–Fe nanoparticle coupled nitrogen-enriched porous carbon (CoyFe10−yOx/NPC) nanosheets as highly efficient non-precious-metal electrocatalysts for the oxygen evolution reaction (OER). Nitrogen-enriched porous carbon (NPC) nanosheets were prepared using a Schiff-base network (SNW) as the precursor and the SNW was based on commercially available and inexpensive monomers which were terephthalaldehyde and melamine. The resulting SNW possessed a high nitrogen content, a high surface area and a high density of metal-coordination sites. In addition, when used as the catalyst for the OER, the Co–Fe nanoparticle catalyst containing 30% Co (Co3Fe7Ox/NPC) showed the highest activity, requiring 328 mV over-potential to achieve a stable current density of 10 mA cm−2 for at least 15 h and a small Tafel slope of 31.4 mV dec−1 in 1.0 M KOH solution, which were comparable even superior to those of many other non-noble metal catalysts. Consequently, the high efficiency and durability make these supported amorphous Co–Fe nanoparticles potentially applicable for improving the performance for electrolysis of water and energy storage applications. More importantly, the support of electrode materials comes from the pyrolysis of porous polymers and this idea offers a new possibility for exploring overall water splitting non-precious-metal catalysts.

Ultrafine Co2P nanoparticles encapsulated in nitrogen and phosphorus dual-doped porous carbon nanosheet/carbon nanotube hybrids: high-performance bifunctional electrocatalysts for overall water splitting

The development of active, robust, and nonprecious electrocatalysts for both the oxygen evolution reaction and hydrogen evolution reaction (OER and HER) is highly crucial and challenging. Herein, ultrafine Co2P nanoparticles (NPs) encapsulated in nitrogen and phosphorus dual-doped porous carbon nanosheet/carbon nanotube hybrids are prepared via a straightforward pyrolysis method. Impressively, the hybrids exhibit remarkable catalytic performance for both the OER and HER in 1.0 M KOH solution, with a current density of 10 mA cm−2 at low over-potentials of 280 mV for the OER and 154 mV for the HER, respectively. More importantly, when fabricated as an alkaline electrolyzer, the hybrids afford 10 mA cm−2 at a cell voltage of 1.64 V with strong stability, rivalling the integrated performance of a commercial IrO2 and Pt catalyst couple. The achieved ultrahigh catalytic performance can be attributed to the nitrogen and phosphorus dual-doped carbon nanosheets, carbon-encapsulated ultrafine Co2P NPs, high conductivity of incorporated carbon nanotubes, large surface area (199.94 m2 g−1), interpenetrated macro-/mesoporous structure, and the strong synergistic effect among these factors.

Cobalt nanoparticles in hollow mesoporous spheres as a highly efficient and rapid magnetically separable catalyst for selective epoxidation of styrene with molecular oxygen

Uniform rattle-type hollow mesoporous spheres (HMS) with large cavities have been successfully prepared by the colloidal carbon spheres as templates. The spheres are well monodispersed and nearly uniform in dimension with particle size of ca. 150 nm. Co nanoparticles composed of HMS were found to be a highly efficient catalyst for selective epoxidation of styrene under mild reaction conditions. The catalysts can be easily separated from the reaction mixtures by an external magnet, and reused five times without significant loss of catalytic activity.

Programmed synthesis of magnetic mesoporous silica nanotubes with tiny Au nanoparticles: a highly novel catalyst system

Magnetic mesoporous silica nanotubes were produced from carbon nanotubes using a well-controlled programmed synthesis method and were characterized by TEM, XRD, XPS, N2 adsorption–desorption and VSM. The well-designed nanotubes had a large specific surface area (1017 m2 g−1), a highly open mesoporous structure (∼3.2 nm) and high magnetization (18.6 emu g−1). Ultrafine gold nanoparticles were successfully supported on the thiol-modified nanotubes by a co-precipitation method. These unique multicomponent nanotubes showed high performance in the catalytic reduction of 4-nitrophenol (with a conversion of 99% in 6 min), and styrene epoxidation with high conversion (65%) and selectivity (58%). Interestingly, the new catalysts could be recovered by magnetic separation from the reaction mixture and could be recycled several times without any significant loss in activity. The unique nanostructure of the nanotubes resulted in a novel, stable and easy to use catalyst system for application in various industrial processes.

Preparation of recoverable Fe3O4@PANI–PdII core/shell catalysts for Suzuki carbonylative cross-coupling reactions

We report on the synthesis, characterization and catalytic performance of a palladium-based superparamagnetic catalyst of Fe3O4@polyaniline core/shell microspheres (Fe3O4@PANI–PdII). The material was characterized by TEM, FT-IR, vibrating sample magnetometry (VSM), XRD, and XPS. The catalyst showed high activity for the carbonylative cross-coupling reaction of aryl iodide with arylboronic acid. Moreover it could selectively reduce the formation of a direct-coupling product. The newly developed catalyst could be recovered from the liquid phase easily by magnetic separation and recycled 5 times without any significant loss of activity.

Controllable orientation-dependent crystal growth of high-index faceted dendritic NiC0.2 nanosheets as high-performance bifunctional electrocatalysts for overall water splitting

Developing highly active, non-noble-metal electrocatalysts for both H2 and O2 evolution reactions (HER and OER, respectively) still remains a great challenge. Herein, we report a facile route for the controllable orientation-dependent crystal growth of {20} high-index faceted dendritic hexagonal NiCx nanosheets on Ni-coated copper foil, with an optimized carbon content of 16.7 at%, by a mild electrodeposition approach. Impressively, the as-prepared material (denoted as d-NiC0.2NS/Ni/CF) shows remarkable catalytic activity (with overpotentials of 121 mV and 228 mV at 10 mA cm−2 for the HER and OER, respectively), simultaneously giving a nearly 100% faradaic yield and affording superior catalytic stability (beyond 100 h) as a bifunctional electrocatalyst for both the HER and OER in basic media. The achieved ultrahigh catalytic performance of d-NiC0.2NS/Ni/CF is primarily attributed to its dendritic nanosheet morphology, optimized 16.7 at% carbon content, and fully exposed {20} high-index facets, which lead to improved mass/electron transport capability and fully exposed active sites. In comparison with the other non-noble-metal electrocatalysts developed to date, we provide a controllable and mild strategy to fabricate high-performance nickel-carbide-based electrocatalysts for advanced applications.

Fabrication of Ag/γ-Fe2O3@TiO2 hollow magnetic core–shell nanospheres as highly efficient catalysts for the synthesis of β-enaminones

Herein, we describe a method to prepare hollow magnetic mesoporous sphere catalysts (Ag/γ-Fe2O3@meso-TiO2). The core–shell strategy efficiently prevents the aggregation of Ag NPs in the high temperature calcination process and the leaching of Ag NPs for the catalytic reaction in the liquid phase. The catalyst is characterized by TEM, XRD and ICP-AES. Moreover, the catalyst exhibited improved activity for the synthesis of β-enaminones, and it could be easily recovered by an external magnet from the reaction mixture and recycled five times without any significant loss in activity.